1. Field of the Invention
The present invention relates to a scale in which a predetermined pattern is formed and an optical displacement detection apparatus that detects a displacement with a head in which a sensor is provided.
2. Description of the Related Art
For example, Jpn. Pat. Appln. KOKAI Publication No. 48-78959 discloses the following technology as a technology relating to the optical displacement detection apparatus.
In the photoelectric detection apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 48-78959, slits or reflecting surfaces are provided in line at constant intervals in a test object, and optical pulses derived from the slits or the reflecting surfaces are detected. The photoelectric detection apparatus is configured such that lengths of the slits or the reflecting surfaces are sequentially increased clockwise.
FIG. 33 is a sectional view illustrating a photoelectric detection apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 48-78959. As illustrated in FIG. 33, the slit of a scale 4 that is of the test object is irradiated with a light beam emitted from a light source 1, and the transmitted light is detected by a photodetector 2.
FIG. 34 is a plan view of the scale 4. As illustrated in FIG. 34, the lengths of the slits provided in the scale 4 are increased or decreased from a reference position at constant intervals relative to a rotating direction indicated by an arrow.
FIG. 35 is a view illustrating an output signal of the photoelectric detection apparatus. In the graph illustrated in FIG. 35, a horizontal axis indicates a rotation angle of the test object, and a vertical axis indicates an output of the photodetector 2. When the scale 4 rotates counterclockwise, an aperture length of the slit is decreased. Jpn. Pat. Appln. KOKAI Publication No. 48-78959 describes that a characteristic in which the amplitude of periodic signal is gradually decreased is obtained as illustrated in FIG. 35.
Jpn. Pat. Appln. KOKAI Publication No. 48-78959 describes that a rotation speed is detected by properly calculating the periodic signal from the photodetector 2, and a change in amplitude is detected, which allows the rotating direction of the scale 4 to be detected.
Although not described in Jpn. Pat. Appln. KOKAI Publication No. 48-78959, in detecting an absolute position by the above configuration, magnitude of an amplitude 104 (see FIG. 36) or a DC component 102 (see FIG. 37) of the output signal is previously checked relative to a rotation displacement such that the absolute position of a rotation angle of the scale 4 can be detected from the reference position by measuring the amplitude or the DC level of the output signal.
That is, the photoelectric detection apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 48-78959 detects the movement direction or the absolute position of the test object based on the amplitude of the detection signal that is changed according to movement of the test object.
In the technology disclosed in Patent Literature 1, as illustrated in FIG. 37, actually an output signal 101 has the DC component 102 that is largely influenced by an optical disposition of the light source 1 or the scale 4. As illustrated in FIG. 37, the DC component 102 is shifted as the amplitude of the detection signal is displaced.
It is assumed that the aperture length of the slit is decreased in order to improve detection sensitivity of the absolute position or to widen an absolute position detection range. In this case, the amplitude of the detection signal is decreased with decreasing minimum value of the aperture length of the slit. Accordingly, in this case, a noise component of the periodic signal from the photodetector 2 is relatively increased, and detection performance (resolution and stability) is degraded at a displacement point where the slit has a small aperture length.
Additionally, when the absolute position is detected by the configuration disclosed in Patent Literature 1, it is necessary to previously check the characteristic of the amplitude 104 of the output signal relative to the rotation displacement. As illustrated by the amplitude 104 and an amplitude 104′ of FIG. 36, the characteristic 104 of the amplitude of the output signal relative to the rotation displacement is changed due to, for example, an ambient environment, attaching looseness of a sensor, and a change with time. Accordingly, the absolute position is detected with significantly low accuracy.
In the technology disclosed in Patent Literature 1, when the detection sensitivity of the absolute position is improved, or when the detection range is widened, the resolution and the stability are significantly deteriorated in both the absolute position detection and the relative position detection.
In view of the foregoing, an object of the invention is to provide an optical displacement detection apparatus in which the improvement of the detection sensitivity of the absolute position and the enlarged detection range are implemented while the resolution and the stability are maintained at a high level in both the absolute position detection and the relative position detection.